A variety of compositions and devices have been used in an effort to inhibit fertilization. Among the contraceptives presently in use are spermicides or inhibitors of sperm function. Of these, the most commonly used are cytotoxic agents, such as nonoxynol-9, a nonionic surfactant. In addition, the use of agents which inhibit enzymes necessary for sperm function has also been addressed. One enzyme which has been targeted for contraceptive action is acrosin. Acrosin is a 38,000 Da plasmin- or trypsin-like proteinase found in the acrosomes of mammalian spermatozoa which degrades the zona pellucida of the ovum, a glyco-proteinaceous wall which must be broken for fertilization to occur. Acrosin activity is absent from newly ejaculated sperm but appears on the spermatozoa in the female reproductive tract after "capacitation," a process by which sperm achieve the ability to penetrate the ovum. The capacitation process involves activation of acrosin and other proteinases, apparently through the removal of enzyme inhibitors present in the Seminal plasma. Release of proteinase inhibitors from the cortical granules of the ovum following fertilization may act as a block to polyspermy (Conrad et al., J. Reprod. Fert. 27: 133-135, 1971). These processes are reviewed by McRorie et al. (Ann. Rev. Biochem. 43: 777, 1974); Hafez, ed. (Human Semen and Fertility Regulation in Men, C. V. Mosby Co., St. Louis, Mo., 1976, pp. 201-242 and 563-582); and Zaneveld et al. (Proc. Soc. Exp. Biol. Med. 133: 1172, 1970).
Certain inhibitors of acrosin have been shown to prevent fertilization in vitro, but only a limited number of these are useful as contraceptives. The use of non-protein proteinase inhibitors, including acrosin inhibitors, in intravaginal or intrauterine contraceptive devices is disclosed by Zimmerman et al. (U.S. Pat. Nos. 4,264,575; 4,264,576; 4,264,577; 4,493,699; and 4,469,671). The proteinase inhibitors disclosed are cationic surfactant salts, such as cationic salts of various carboxylic and sulfonic, sulfuric, or phosphoric acids of relatively high molecular weight hydrocarbon moieties. The nature of the interaction(s) between these compounds and the target enzymes is not disclosed. In vitro treatment of capacitated rabbit sperm with pancreatic trypsin inhibitor or partially purified seminal plasma inhibitor inhibited fertilization when the treated sperm were introduced into the oviducts of female rabbits via injection (Zaneveld et al., J. Reprod. Fert. 225: 387-392, 1971). Zaneveld et al. (FEBS Lett. 11: 345-347, 1970) demonstrated that soybean and lima bean trypsin inhibitors block acrosin activity and prevent fertilization in vitro. They also demonstrated the use of the non-protein acrosin inhibitor tosyl-L-lysine chloromethyl ketone (TLCK) as a vaginal contraceptive in rabbits.
In contrast to non-protein proteinase inhibitors, naturally-occurring protein proteinase inhibitors associate reversibly with their substrates (Tschesche, Angew. Chem. Int. Ed. Eng. 13: 10-28, 1974, and Zaneveld et al., 1970, ibid.) and would therefore not be expected to be effective as contraceptive agents given that the normal process of sperm capacitation in the female reproductive tract involves the removal of protein inhibitors of acrosin.
One such naturally-occurring protein is alpha-1-antitrypsin (also known as alpha-1-proteinase inhibitor), which has been shown to inhibit human and boar acrosin in vitro (Fritz et al., Hoppe-Seyler's Z. Physiol Chem. 343: 1950-1952 and 1953-1956, 1972). Since this inhibitor is generally believed to function through competitive (reversible) inhibition of the substrate (Carrell et al., Nature 298: 329-334, 1982),i it would riot be expected to be suitable for contraceptive use. Furthermore, cervical secretions of human females have been shown to contain inhibitors of trypsin and chymotrypsin (Haendle et al., Hoppe-Seyler's Z. Physiol. Chem. 351: 545-546, 1970).
During the decade 1973-1982 (the most recent period for which extensive data are available), the use of oral contraceptives among women in the United States sharply declined (Bachtach, Fam. Plann. Perspect. 16: 253-259, 1984). This may be traced, at least in part, to complications associated with the use of oral contraceptives, including an increased risk of cancer, blood clots, and associated disorders. Use of "the pill" has therefore been supplanted by use of less effective barrier methods, such as condoms, diaphragms, and foams and by sterilization. Another alternative, the intrauterine device (IUD), has also been associated with health risks to the user. Many of these devices have been withdrawn from the marketplace. The traditional barrier methods often rely in part on spermicidal agents for their effectiveness. In general, only a limited number of sperm-inhibiting compounds have proven to be acceptable as contraceptive agents. The most widely used spermicidal agents are harsh, cytotoxic chemicals which may cause irritation or allergic reactions in users. Further, it has been suggested that the toxic effects of these agents on sperm may lead to birth defects when partially impaired sperm fertilize the ovum.
There is thus a need for impermanent, dependable methods of contraception which are not associated with the health risks to the user or her offspring described above. The present invention provides such a method while further providing other related advantages.